Examples of ophthalmologic apparatuses for capturing an image of the fundus or the anterior segment of the subject's eye include an apparatus using optical coherence tomography (OCT), a fundus camera, a scanning laser ophthalmoscope (SLO), a slit lamp, and the like. Among them, OCT has been drawing attention. OCT creates an image representing the exterior structure, interior structure, or the like of an object to be measured using light beams from a laser light source or the like. Unlike X-ray computed tomography (CT), OCT is not invasive on the human body, and therefore is expected to be applied to the medical field and the biological field, in particular. For example, in the ophthalmological field, apparatuses for forming images of the fundus oculi or the cornea have been in practical use.
Patent Document 1 discloses a device using Fourier-domain OCT or frequency-domain OCT. This device irradiates an object to be measured with a beam of low-coherence light, and superimposes the light reflected from the object on reference light to generate interference light. The device then obtains the spectral intensity distribution of the interference light, and applies Fourier transform thereto to acquire an image of the morphology of the object to be measured in the depth direction (z direction). The device includes a galvanometer mirror configured to scan a light beam (signal light) in a direction (x direction) perpendicular to the z direction, thereby forming an image of a desired area of the object to be measured. The image formed by the device is a two-dimensional tomographic image in the depth direction (z direction), taken along the scanning direction (x direction) of the light beam. This technique is also called “spectral-domain”.
Patent Document 2 discloses a technology, in which signal light is scanned in the horizontal direction (x direction) and the vertical direction (y direction) to thereby form a plurality of two-dimensional tomographic images in the horizontal direction. Based on the tomographic images, three-dimensional tomographic information is acquired for a measurement range. As the three-dimensional imaging, for example, there are a method of displaying a plurality of tomographic images arranged in the vertical direction (referred to as “stack data”, etc.), a method of performing rendering on volume data (voxel data) generated based on the stack data to thereby form a three-dimensional image, and the like.
Patent Documents 3 and 4 disclose OCT devices of other types. Patent Document 3 discloses an OCT device, which scans (sweeps) the wavelengths of light irradiated to the object to be measured, and sequentially detects interference light obtained by superimposing reflected light of each wavelength on reference light to acquire spectral intensity distribution. The device applies Fourier transform to the spectral intensity distribution to form an image of the morphology of the object to be measured. Such an OCT device is called swept source OCT. The swept source OCT is a type of Fourier-domain OCT.
Patent Document 4 discloses an OCT device, which irradiates light beams having a predetermined diameter to an object to be measured, and analyzes the component of interference light obtained by superimposing the reflected light on reference light. Thereby, the device forms an image of the object to be measured in a cross-section perpendicular to the traveling direction of the light. Such an OCT device is called full-field OCT or en-face OCT.
Patent Document 5 discloses a configuration in which OCT is applied to the ophthalmologic field. Incidentally, before the application of OCT, a fundus camera, a slit lamp, SLO, or the like has been used as a device for observing the subject's eye (see, for example, Patent Documents 6, 7, and 8). The fundus camera is a device that irradiates the subject's eye with illumination light and receives the light reflected from the fundus to thereby capture an image of the fundus. The slit lamp is a device that cuts an optical section of the cornea using a slit light to thereby acquire an image of the cross-section of the cornea. The SLO is a device that scans the fundus with a laser beam, and detects the reflected light with a high-sensitivity element such as a photomultiplier tube for imaging the morphology of the fundus surface.
The devices using OCT offer advantages with respect to the fundus camera in that they can acquire high-resolution images, and also that they can obtain tomographic images as well as three-dimensional images.
As described above, the devices using OCT can be used for the observation of different parts of the subject's eye, and are capable of acquiring high-resolution images. Therefore, the OCT devices have been used in diagnosing a variety of ophthalmologic diseases.
Among those using OCT, there has been a known ophthalmologic apparatus configured to capture a high-resolution image regardless of the involuntary eye movement during fixation (flicks) of the subject's eye (see, for example, Patent Document 9). Besides, there has been a known method for obtaining misregistration between two images using a phase only correlation function (see, for example, Patent Document 10). For more information regarding the phase only correlation function, reference may be made to, for example, Non-Patent Document 1.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. Hei 11-325849    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-139421    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2007-24677    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2006-153838    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2008-73099    [Patent Document 6] Japanese Unexamined Patent Application Publication No. Hei 9-276232    [Patent Document 7] Japanese Unexamined Patent Application Publication No. 2008-259544    [Patent Document 8] Japanese Unexamined Patent Application Publication No. 2009-11381    [Patent Document 9] Japanese Unexamined Patent Application Publication No. 2011-212103    [Patent Document 10] Japanese Unexamined Patent Application Publication No. 2010-110391